Spot Tests Again

Question:

Is there an easy method for determining the metal in a plated deposit? We have to strip deposits from steel-based substrates and find that we sometimes have difficulty identifying the plated deposits. S.W.

Answer:

I am asked this question quite often. There are a number of places where you can find this information.

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The following sequence that is taken from SOLUTION 4.1 Software by Lawrence J. Durney, Kushner Electroplating School, 408-749-8652, is a straightforward approach to determining the makeup of the plated layer.

I. Colored plating: add 1 drop 75% nitric acid.

A. Blue solution - copper, brass or other copper alloys
B. Reaction delayed or does not occur - gold

Note: A chromate coating can sometimes mimic brass. However, with the addition of the nitric acid, the coating will usually bleach out. The drop will then usually turn yellow or green, and the surface will become white.

II. Colorless plating: add nitric acid and wait 2 min if the reaction is slow.

A. Plating is not attacked: add 1 drop of sodium hydroxide to new spot.

1. Plating is attacked - aluminum
2. Plating is not attacked - add concentrated hydrochloric acid.

a. Green solution - chromium
b. No action - rhodium, platinum

B. Plating attacked by nitric acid giving a green solution.

1. Place HCl on a new spot.
2. Take up on filter paper, make ammoniacal and add dimethyl glyoxime. This solution should be reasonably fresh (less than 6-8 weeks old) and be stored in a dark bottle to retard deterioration.

a. Red coloration formed - nickel
b. No red coloration - chromium

C. Plating attacked giving a colorless solution. Place a small amount of solid cacotheline on a small filter paper and add 1 drop of water. Place 1 drop of HCl on the metal and touch the reverse side of the paper holding the cacotheline to the spot of acid. A red violet color is a positive reaction.

1. Reaction positive - tin
2. Reaction not positive - place nitric acid on a new spot, take up on filter paper, make ammoniacal, add ammonium sulfide.

a. Spot becomes colorless or white - zinc
b. Spot becomes bright yellow - cadmium
c. Brown-black spot - nitric acid on new spot, take up on filter paper, add NaOH.

i. Brown-black spot – silver
ii. Colorless or white – lead